Battery operated industrial trucks have at least two drive units, i.e. one for the travelling operation and the other one for the lifting/lowering operation of the load support. As driving motors, rotary current motors are mostly used, and the actuation of the at least one driving wheel and that of the load supports takes place either directly by the electric motor, or via a hydraulic unit driven by the electric motor. For the lifting and lowering operation, the electric motor drives a hydraulic pump. For the travelling operation, at least one electric motor drives the driving wheel or several driving wheels directly or via gearings, for instance. Further, it is known since a longer time to regenerate energy in the braking and/or lowering operation of the industrial truck, and to feed it back into the battery. In these cases, (regenerative braking or regenerative lowering, respectively), the electric motors work as generators.
In the operation mode as a generator, excess voltages are generated, depending upon the dimension of the battery, the kind of battery and the amount of back-fed energy. In fact, these excess voltages take place only temporarily, for some seconds, e.g., but can act in a destructive manner for the electronic components of the control or regulation device for the driving motors, or for other components connected to the battery. It is therefore obligatory to provide a protection for this case.
It is known to carry out a kick-off switching in the case of excess voltages. This precaution has the disadvantage that the availability of the industrial truck is decreased. Further, it is conceivable to install a loss resistance for the reduction of excess voltages, along which the destructive energy is dissipated. However, this measure increases the expense for the electronic components.
From DE 102 44 769A it is known to conduct the electric energy, which is generated upon braking down a mobile working machine, to another electric drive.
Thus, the present invention is based on the objective to create a battery operated industrial truck in which excess voltages are reduced in a particularly simple and effective manner.